In particular in the field of hip endoprosthetics, the cone insertion connection between the femoral head and the hip-stem has proven itself as a stable self-locking clamped connection. Moreover, on account of the clamping properties thereof, cone insertion connections are also used in other prosthetics care systems.
However, problems arise in relation to renewing the cone insertion connection when one of the prosthetics components of the prosthetics system needs to be replaced.
For example, replacing the joint head is generally associated with altering the ball socket. In such a case, in order not to have to also explant the existing shaft and replace said shaft with a shaft that corresponds to the cone of the new joint head, adapter sleeves have been developed, by means of which the new joint head can be securely connected, in situ, with the remaining shaft, by means of a cone insertion connection.
A plurality of conical adapter sleeves is known from the prior art, which sleeves have been designed for various purposes.
DE 40 08 563 A1 discloses a cone insertion connection of this kind, in which a conical adapter is used as a coupling means between the joint head and the shaft neck in order to be able to connect joint heads and shaft necks by means of a frictional connection, it being possible for the joint and the shaft neck to be produced from different materials and to also have different conicities.
Pre-assembled adapters for cone insertion connections are known from CH 676 922 A5 and WO 2005/012596 A1. Said latter document in addition discloses the production of a conical adapter of this kind from biocompatible plastics materials.
It is also known that, after the joint head has been removed, the surface a shaft cone no longer has the quality that allows it to be pushed onto a replacement head in a manner free of stress concentrations. As a result of said stress concentrations, fractures and breaks may occur in the joint head, which can in turn lead to renewed altering of the replacement joint head.
This problem is addressed in DE 103 03 660 B4, which document specifies an adapter for providing existing shafts with new joint heads, which adapter ensures that the stresses are distributed uniformly over the contact surface or are diverted at least into the upper region of the cone. For this purpose, the adapter comprises a groove that extends upwards from the lower, wider face of said adapter, and the female taper of said adapter has a curved course.
DE 199 04 437 A1 describes a conical adapter that has particular elasticity and damping properties. In order to more uniformly divert the stress between the cone of the shaft and the ball head, the adapter is formed as a porous package. The disclosed design makes it possible to replace damaged ball heads without it being necessary to remove the shaft, even if the state of the surface of the cone would no longer allow the ball heads to be placed on directly.
In order to compensate for winding errors in the cone on the prosthesis neck while at the same time increasing the useful life of the prosthesis, EP 2 459 124 B4 provides an adapter that, although fundamentally a conical molded body, nonetheless comprises resilient and bearing portions which divert the force into a defined region in the joint head and thus relieve the load-bearing regions.
A problem in prostheses that bear a cone is that the cone geometries are not standardized, and therefore the dimensions of the cones of different manufacturers may differ significantly from one another. Specifications such as “12/14 Euro cone” or “12/14 standard cone” likewise do not denote a cone geometry that is subject to a standard and which guarantees standardized dimensions irrespective of the manufacturer.
If the cone of the prosthesis is then additionally damaged or if the tolerances thereof are outside the standard deviation usual in production, it is no longer possible to ensure a secure and long-term stable cone insertion connection between the prosthesis components, even when using a cone adapter.